Sweat the onions in the olive oil for 6 to 8 minutes until soft and translucent. Add the remaining ingredients and spices except for garnishes. Bring to a boil and simmer gently for 25 minutes until the potatoes, lentils, and carrots are soft. Let the soup cool a little and blitz it with an immersion blender until smooth and creamy. If you want to add some cream or half and half, by all means go ahead, but you don't need to. TASTE the soup. Add more harissa, spice, salt and pepper as needed. Ladle soup into bowls and garnish with Nigella seeds, a dollop of Greek yogurt, and some cilantro.